@gap2368 has a good starting strategy - by taking him to local venues (like the zoo, State/County Fair, busy stores/malls, festivals, etc.) you can help acclimate him to some of the conditions that he will encounter at WDW. Just practicing staying seated is a great place to start. If he gets "rabbit fever" and hops out of his chair at WDW, he could easily get bowled over by another Guest who wasn't expecting him to jump out of the chair.
Since he has his own chair, you don't have to worry about getting the "stroller as a wheelchair" tag at Guest Relations. If you decide to ask for a
DAS for him, remember that you will focus on WHAT his symptoms are and HOW the DAS helps him in the Parks, NOT the diagnosis. There's *tons* of DAS-related threads here; go to the top right corner of any page and type in DAS next to that magnifying glass in the "search" box!
And don't forget to use the resources that Sue and the other mods have "pinned" to the top of this forum's main page - there's a *lot* of good information there for you.
Don't worry too much about what rides, attractions and shows to do or not do - unless you have a specific issue (for example, strobe lights cause seizures, or claustrophobia causes a catastrophic reaction, etc.) everything at WDW is made to be enjoyed by people of all ages. It's still amazing to me - the first time I went to WDW was in July 1972, at the age of 12. As a kid, I saw the Magic Kingdom through completely different eyes than my parents did. If you had asked our family about just one ride afterward, my brother and I would have had an entirely different experience than our parents did! And that's OK; it's meant to be that way!
If I was going to say "avoid anything", it would probably be things like Hall of Presidents & Carousel of Progress at MK, or American Adventure at Epcot. Those kinds of shows aren't going to be as interesting or visually stimulating for him, and let's face it... a stage full of Animatronic Presidents is a great place for adults to sit and cool off on a hot day... but most likely a form of torture for wiggly little guys (and girls!)
You - his family - knows him better than anyone in the world. If he gets overstimulated easily, then plan your days with breaks at the Resort hotel, and don't pick an over-themed room so that everyone has a visual "break" from all the Disney-ness. If visual stimulation isn't a problem, but auditory issues are (too much sound, crowd noise, sudden noises, fireworks, etc.) then practice wearing some of the headphones or ear plugs that others here have found for their kids. If he has any touch issues - fabric sensitivities, or texture issues, you may want to consider how to mitigate those, especially for the fur character meet-n-greets.
If he's one of those kids that is just like a sponge - he will soak it all up as long as someone can keep up with him - then start by showing him some
YouTube videos that people have posted of rides, attractions and shows. Frequently remind him that there will be LOTS of people who want to meet Mickey Mouse and all the other characters, so we have wait our turn! At his age, don't overwhelm him, but maybe show him one video a day, but let him watch it several times so that he gets a chance to really learn and absorb what he will see when you are there. Don't worry about ruining the trip; lots of kids do better if they know what to expect before they ride Peter Pan or 7DMT!
The main thing I would NOT do to prepare him (or any other family member, for that matter) is to show him Disney's own videos. In their marketing materials, it looks like the Parks are not at all crowded... everyone gets a front-row spot for everything! And we never see anyone pay for any of the food, drink, or souvenirs that the family is happily consuming. Disney produces beautiful, heartfelt marketing... but it bears no resemblance to the reality you will encounter.
Last but not least, please don't despair if you get there, and it seems like all he wants to do is ride the same few things over and over, and eat the same foods over and over. That's OK! If that's the *worst* thing that happens - you are still at Disney World! Take your lead from him, and try to see WDW through his eyes, and you'll both find the Magic!
